A common method of forming single-layer stator windings in large rotating electrical machines is to slot a plurality of pre-formed coils into open-type winding slots formed in a surface of the stator. The coils are formed separately from the stator and then individually dropped into position in the winding slots during the winding process. Each pre-formed coil is constructed from one or more insulated conductors as a complete loop and has several turns. The coils are usually formed from rectangular conductors but may also be formed from round conductors as long as the complete coil is substantially rectangular in cross-section. A typical individual pre-formed coil is shown in FIG. 1. The pre-formed coil 2 has two axially-extending winding runs 4 that are each received within an open-type winding slot formed in the stator of the electrical machine. The pre-formed coil 2 has an endwinding 6 at each end that protrudes out from the axial end of the stator when the coil is positioned within the winding slots. The axially-extending winding runs 4 and endwinding 6 of the pre-formed coil are both substantially rectangular in cross-section in order that the coil may be received in the winding slots and function properly.
In electrical machines with single-layer windings each winding slot in the stator contains only one winding run 4 and the two winding runs of each pre-formed coil are spaced so as to be received in winding slots that are separated by a number of intermediate winding slots. For example, pre-formed coils for a three-phase single-layer winding with one-slot-per-pole-per-phase have a pitch of three winding slot pitches. This means that the two axially-extending winding runs of each coil are received in winding slots that have two intermediate winding slots between them.
Conventional rotating electrical machines use stator windings with a plurality of pre-formed coils, as described above. The stator will include a number of identical winding slots that extend radially into the laminated stator core and are uniformly spaced around the circumference of the stator. The winding slots will often be “open-type” that are substantially parallel-sided along their length and depth in order that the pre-formed coils can be inserted into them in a radial direction. The winding slots will normally be formed parallel to the longitudinal axis of the stator, but in some electrical machines they may be formed at an angle to the longitudinal axis of the stator so that the magneto-motive force (mmf) harmonics can be reduced.
Winding slots that extend into the stator along a radius of the stator are generally preferred because they are simple to form and provide suitable stator teeth characteristics. For the purposes of the following description such slots are referred to as “radial slots”. The stators of most large rotating electrical machines are usually formed from a plurality of axially stacked laminations in which the winding slots are formed in each individual lamination by punching. In many cases this is done one slot at a time by an indexing punching machine and this process is greatly facilitated if the slots are radial slots. If two-layer stator windings are used then it is necessary to have radial winding slots in order to allow all the pre-formed coils to be substantially identical in shape. Radial slots also maximise the circumferential width of the stator teeth formed between the winding slots at all points along the radial length of the teeth. As a consequence, the generation of undesirable flux densities in the stator teeth during operation of the machine and the mmf needed to establish that flux density are both minimised. Furthermore, having radial slots and thereby maximising the circumferential width of the teeth also maximises the structural strength and rigidity of the stator teeth that are formed between each pair of adjacent winding slots and ensures that they can withstand the stresses they are subjected to during the winding process and during operation of the electrical machine.
However, in order to insert the pre-formed coils into radial slots it is necessary that the two winding runs of each coil are formed to be in the same orientation as the radial slots. In other words, the axially-extending winding runs of each pre-formed coil must be formed such that, when they are received in the winding slots, they are parallel to one another in the axial direction of the stator and each lies along a different radius of the stator. This means that the winding runs of each pre-formed coil will be angled relative to one another. More specifically, in a conventional rotating electrical machine the two axially-extending winding runs of each coil must be formed at an angle relative to one another that is equal to the circumferential angular separation between the central planes of the winding slots into which the winding runs are to be received. The central plane of an open-type winding slot is defined here as the plane that is equidistant from, and parallel to, the sides of the winding slot. The central plane of a radial slot will define the angular centre of that slot about the circumference of the stator.
Rotating electrical machines are typically formed such that the stator is situated radially outside of the rotor such that the winding slots are formed in the radially inner surface of the stator. However, it is also possible to form rotating electrical machines with the rotor formed radially outside of the stator such that the winding slots are formed in the radially outer surface of the stator. The axially-extending winding runs of each pre-formed coil must be angled towards each other or away from each other as required in order to be properly received in the winding slots of the corresponding winding slot pair.
FIG. 2 shows a section of a stator 8 for a large conventional rotating electrical machine. The complete stator 8 contains 240 open-type radial slots 10 that are uniformly spaced around the circumference of the radially inner surface of the stator. A single-layer pre-formed coil 2 of one slot-per-pole-per-phase construction is received in the winding slots 10a and 10d that together define a winding slot pair. The angular separation between the central planes of each pair of adjacent winding slots 10 is 1.5° (i.e. 360°/240). Each slot 10 extends substantially parallel to the longitudinal axis of the stator 8 and extends into the radially inner surface of the stator along a radius of the stator. As the slots 10 are open-type slots they are substantially parallel-sided, thus enabling a winding run of each pre-formed coil to be inserted into each slot along a radius of the stator. The pre-formed coil 2 has a pitch of three slots and the winding slots 10a and 10d that receive the winding runs 4 are therefore separated by two intermediate winding slots 10b and 10c. The angular separation between the central planes of the winding slots 10a and 10d is 4.5°. Therefore, as each winding slot 10 extends along a radius of the stator, the axially-extending winding runs 4 of the pre-formed coil 2 are also angled relative to each other by 4.5°.
Stator teeth 14 are formed between each pair of adjacent winding slots 10. As a result of the orientation of the winding slots 10, the stator teeth 14 are all oriented along a radius of the stator and are substantially identical. Furthermore, as the winding slots 10 are of uniform width, the stator teeth 14 are slightly wider at their base than at the radially inner surface of the stator 8. Stator teeth 14 formed in this manner are suitably strong and have minimal undesirable flux densities generated within them during operation of the electrical machine.
A stator that receives a single-layer stator winding will normally be wound by individually inserting the pre-formed coils into the winding slots. However, the need for the axially-extending winding runs of each pre-formed coil to be formed at an angle relative to one another so that each winding run extends along a radius of the stator can make it difficult to insert the coils into the radial slots. This means that the winding process is often time-consuming and may require a high degree of skill in order to prevent the pre-formed coils being damaged. These problems are particularly acute in large electrical machines with a high number of poles as the pre-formed coils for such electrical machines are also large and their endwindings are inherently stiff. Accordingly, there is a need for an improved stator with winding slots that makes the insertion of pre-formed coils during winding much easier. There is a particular need for such a stator that is suitable for large multi-phase rotating electrical machines with high numbers of poles. However, it is important that any such stator generates acceptable limits of undesirable flux densities during operation of the electrical machine and has stator teeth formed between its winding slots that are sufficiently strong.